FIG. 14 is a simulated view for explanation of the conventional technology. In FIG. 14, designated at the reference numeral 1 is a laser oscillator, at 2 a laser beam oscillated from the laser beam 1, at 3a, 3b bend mirrors each for bending the laser beam 2 oscillated from the laser oscillator 1, at 4 an image transferring mask having an aperture smaller than that of a diameter of the laser beam in a central section thereof, at 5 a galvano-mirror, at 6 a transmission type of optical component such as an f-.theta. lens, and at 7 a wiring board as an object to be machined.
In FIG. 14, a laser beam 2 oscillated from the laser scillator 1 is bent by the bend mirror 3a and goes into the image transferring mask 4. At this point of time, the laser beam 2 has a diameter larger than the aperture diameter of the image transferring mask 4 and is taken out by a desired amount of energy or as a desired form of beam through the image transferring mask 4. The laser beam 2 having passed through the image transferring mask 4 is bent by the bend mirror 3b, guided to a specified position of the f-.theta. lens 6 by the galvano-mirror 5, and also guided to the wiring board 7, whereby a hole is pierced through the wiring board 7.
It is well known that a laser beam has a different intensity distribution, as indicated by reference signs .alpha., .beta., .gamma. shown in FIG. 15A, according to a position after the laser beam is passed through the lens. Namely, FIG. 15A shows a laser beam 2 at each of the positions indicated by the reference signs .alpha., .beta., .gamma. after the beam is passed through the f-.theta. lens 6, FIG. 15B shows a beam intensity distribution at each of the positions such as at a position indicated by .alpha., namely a focal position, at a position indicated by .beta., and at a position indicated by .gamma. each shown in FIG. 15A, and FIG. 15C is a view showing a form of a machined hole in the wiring board 7 at each of the positions indicated by .alpha., .beta., .gamma. shown in FIG. 15A according to each beam intensity.
Generally, in a case where hole-piercing is executed in the wiring board 7, the wiring board 7 is placed at a position indicated by .alpha. in FIG. 15A, namely at a focal position. In this case, the intensity distribution of the laser beam 2 is flat as shown in FIG. 15B, and as a result, it is possible to machine a hole with a high degree of roundness and is straight in the direction of the board thickness of the wiring board 7 therein as indicated by .alpha. in FIG. 15C.
On the other hand, in a case where the wiring board 7 is placed at a position indicated by .beta. in FIG. 15A, this situation is called a defocusing state, and the laser beam 2 is irradiated to the wiring board 7 as indicated by .beta. in FIG. 15B with the intensity distribution thereof like that by the laser beam 2 with reduced converging characteristic, and for this reason it can be recognized that a tapered hole in which the diameter is large in the upper side of the hole and is small in the lower side thereof is formed, as indicated by .beta. in FIG. 15C.
Further, in a case where the wiring board 7 is placed at a position indicated by .gamma. in FIG. 15A, the intensity distribution of the beam is like that by .gamma. in FIG. 15B, and a tapered angle of the machined hole can be increased as indicated by .gamma. in FIG. 15C. As described above, it is possible to easily change a tapered angle of a machined hole according to increase or decrease of a defocusing rate, so that in recent years, this method has become more and more popular and been used for actual industrial purposes.
In a case where the board is machined in the defocusing state, however, a degree of roundness of a machined hole therein is degraded due to astigmatism in a transmission type of optical components such as an f-.theta. lens or the like.
It is also difficult to vertically adjust the axis of the laser beam after passing through the f-.theta. lens to the wiring board in the area covering all inciding positions of the laser beam to the f-.theta. lens, and for this reason, a positional displacement of a machined hole is generated even if the identical position is tried to be machined in a case of machining the hole at the focal position as well as in a case of machining it in the defocusing state. Accordingly, it is required to correct a positional displacement each time when the defocusing rate is changed, which causes increase of needless works.